Analysis of SiC grain size variation and NaCl concentration on the corrosion susceptibility of AA1070 aluminium matrix composites

Analysis of the impact SiC grain size at 10% wt. on the corrosion resistance of AA1070 aluminium matrix composites was studied in Cl anion containing dilute H2SO4 solution. Studies were performed with potentiodynamic polarization and open circuit potential (OCP) measurement. Optical microscopic studies were performed on the corroded matrix composites. Results show that the corrosion susceptibility of the composites increased proportionately with increase in Cl ion concentration. The matrix composites at 3 um and 9 um SiC grain size exhibited the lowest corrosion rate values in comparison to the base alloy and composites with larger SiC grain sizes. OCP values for the matrix composite at 0 μm SiC in the acid solution (0.25 M H2SO4/0% and 0.25% NaCl) was the most electronegative due to lower thermodynamic stability than matrix composites with larger SiC grain sizes. Optical microscopic studies show that corrosion was mainly localized for all specimens tested due to pitting at low Cl ion concentration in comparison to general corrosion for the metal substrate and total destruction of the matrix composite morphology at higher chloride concentration. Results of statistical analysis through ANOVA showed that SiC grain sizes are more ABOUT THE AUTHORS Roland Tolulope Loto is a professor at the Department of Mechanical Engineering, Covenant University, Ota, Ogun State, Nigeria. He is the principal investigator of the corrosion and materials research cluster. Roland has over 93 research publications including reviews in top international journals and has consistently served as reviewer in respectable journals due to his in-depth knowledge and technical expertise. Roland is a registered member of the Council for the Regulation of Engineering in Nigeria, South African Institute of Mining and Metallurgy and the Corrosion Institute of Southern Africa. Philip Babalola (PhD) is a lecturer at the Department of Mechanical Engineering, Covenant University, Ota, Ogun State, Nigeria. He specializes in research on composites, renewable energy applications, pipe network analysis with significant publications. He is a registered member the Council for Regulation of Engineering in Nigeria and corporate member of the Nigerian Society of Engineers. PUBLIC INTEREST STATEMENT The economic impact and problems resulting from corrosion has drawn strong attention from scientists and engineers worldwide. Corrosion of aluminium alloys in industrial environments is a major concern in chemical processing plants, oil and gas industry, manufacturing and automobile industry, marine operations, boiler and power generation plants due to the considerable cost involved in the replacement of metallic parts in their various applications. The consequence often leads to plant shutdowns, breakdown of industrial equipment, reduced efficiency, industrial downtime, high maintenance cost due to replacement of damaged part, wastage of valuable resources and expensive overdesign. Corrosion prevention through modification of the metallurgical structure of aluminium with the addition of silicon carbide is of great practical importance and can be extensively employed in curtailing wastage of engineering materials and minimizing costs of corrosion control. Loto & Babalola, Cogent Engineering (2018), 5: 1473002 https://doi.org/10.1080/23311916.2018.1473002 © 2018 The Author(s). This open access article is distributed under a Creative Commons Attribution (CC-BY) 4.0 license. Received: 06 February 2018 Accepted: 01 May 2018 First Published: 21 May 2018 *Corresponding author: Roland Tolulope Loto, Department of Mechanical Engineering, Covenant University, Ota, Ogun State, Nigeria E-mail: tolu.loto@gmail.com Reviewing editor: Catherine MacKenzie, Taylor and Francis Group, UK; Guglielmo Lomonaco, Università degli Studi di Genova, Italy Additional information is available at the end of the article


PUBLIC INTEREST STATEMENT
The economic impact and problems resulting from corrosion has drawn strong attention from scientists and engineers worldwide. Corrosion of aluminium alloys in industrial environments is a major concern in chemical processing plants, oil and gas industry, manufacturing and automobile industry, marine operations, boiler and power generation plants due to the considerable cost involved in the replacement of metallic parts in their various applications. The consequence often leads to plant shutdowns, breakdown of industrial equipment, reduced efficiency, industrial downtime, high maintenance cost due to replacement of damaged part, wastage of valuable resources and expensive overdesign. Corrosion prevention through modification of the metallurgical structure of aluminium with the addition of silicon carbide is of great practical importance and can be extensively employed in curtailing wastage of engineering materials and minimizing costs of corrosion control.

Introduction
Corrosion resistance of aluminium alloys is the product of interfacial oxide evolution; however, the oxide is subject to localized breakdown resulting in pitting and general and crevice corrosion of the substrate metal. The major limitation in the application of aluminium alloys for high-performance tribological application in extreme environments is their poor wear properties for engineering applications such as low hardness, poor degradation resistance and weldability, thus the need to develop more efficient materials with greater efficiency without reducing the strength to weight ratio (Sinclair & Gregson, 1997). Aluminium silicon matrix composites (Al-SiC) are new generation of engineering materials for applications such as in automobiles, aeronautics, electronics, recreation and military industries (Chawla, 2013;Lees, Dhihra, & McCullough, 2005;Surappa, 2003). They consist of SiC grains scattered within aluminium alloy substrate. The desired mechanical properties of aluminium and silicon are admixed to give matrix composites with superior strength, higher specific modulus, enhanced rigidity, lighter weight, lower coefficient of thermal expansion, higher thermal conductivity, customized electrical properties, superior abrasion and damping properties than their unreinforced counterparts (Mahendra, Arulshri, & Iyandurai, 2013;Neelima, Mahesh, & Selvaraj, 2011;Singh & Singla, 2012;Singla, Dwivedi, Singh, & Chawla, 2009). Matrix composites are basically an admixture of contrasting metals and intermetallic compounds ingrained within the substrate metal. The characteristics of the composites is subject to the individual characteristics of the intermetallic phases, their corresponding volume, grain dimension and orientation within the matrix (George, Totten, & Scott, 2003;William, 2007). They transmit applied loads to scattered reinforcements secured with the matrix (Pai, Rajan, & Pillai, 2004). Silicon carbide (SiC) is a chemical combination of silicon and carbon elements. It possesses excellent resistance to acids, alkalis and liquefied NaCl till 800°C. It forms a chemically resistant silicon oxide film in air at 1,200°C, with appreciable applicability till 1,600°C. Dispersion of SiC substituents within the matrix material is dependent on properties such as deformation and flow of the molten matrix, technique of grain incorporation and their interaction during the admixture process (Hashim, Looney, & Hashmi, 2002).
The major drawback of Al-SiC is the extent to which the intermetallic constituents affect the resistance of the aluminium matrix to interfacial degradation. Addition of SiC can result in the presence of discontinuities, inclusions and defects in the protective film exposing more areas to corrosion (Trowsdale et al., 1996). The resulting microstructural heterogeneity increases the corrosion susceptibility of Al-SiC due to galvanic effect and interfacial issues. Their excellent heat conduction and small expansion coefficients have resulted in applications which expose them to astringent conditions (Bakkar & Neubert, 2007;Candan & Biligic, 2004;Griffiths & Turnbull, 1994). Processing specifications also significantly affect the composite microstructure and, consequently, their corrosion behaviour. Previous research on modification of SiC content on aluminium matrix composite in H 2 SO 4 solution showed that the presence of SiC at low chloride concentration exposed the matrix material to greater corrosion attack (Loto & Babalola, 2017). Result also showed that SiC at the lowest content of 10% had the highest corrosion resistance in acid chloride solution. This investigation focuses on the effect of SiC grain size variation with respect to chloride concentration on the corrosion susceptibility of AA1070 aluminium. The mechanical properties of aluminium alloys infused with nonmetallic particulates are influenced by the grain size and the volume fraction. Due to lack of improvement in strength and porosity of Al-SiC, when the volume fraction of silicon carbide exceeds 10%, this research is limited to 10% silicon carbide content added in the aluminium alloy as reinforcement.

Materials and preparation
AA1070 aluminium metal (AA1070) was purchased from Aluminium Rolling Mills, Ota, Ogun State, Nigeria. Energy-dispersive spectrometer analysis at Materials Characterization Laboratory, Department of Mechanical Engineering, Covenant University, Ota, Ogun State, Nigeria, gave ostensible (wt.%) constitution shown in Table 1. Silicon carbide (SiC) with ostensible (wt.%) constitution shown in Table 2 and of 320 grit size was sourced from Logitech, UK, and added to AA1070 at weight percentages of 10%. A tilting furnace of 20 kg capacity was used to melt AA1070 for 45 min at 650ºC. SiC was added to molten AA1070, stirred, shaped into moulds (AA1070/SiC) and cooled for 24 h. AA1070/SiC was sectioned mechanically and metallographically prepared using silicon carbide grinding papers (80, 320, 600, 800 and 1,000 grit) and cleansed with deionized H 2 O and propanone for electrochemical tests. Analar grade sodium chloride purchase from Titan Biotech, India, was produced in proportional volumes of 0%, 0.25%, 0.5%, 0.75% and 1% in 200 mL of 0.25 M H 2 SO 4 solution, prepared from standard grade of H 2 SO 4 acid (98%) with deionized H 2 O. Microhardness test, tensile test and electrical conductivity and resistivity tests were performed on AA1070/SiC specimens as shown in Table 3. Microhardness test of AA1070/SiC specimens were performed with LECO 700AT microhardness tester with a load of 492.3mN and a waiting time of 10 s (ASTM Standard E 384). Six tests were performed for every specimen and the mean value was used as the microhardness value. Instron (Model 3369) Universal Testing Machine with 30 kN load (ASTM International E8/E8M-09) was used to perform tensile test on AA1070/SiC specimens. The test specimens with mean cross-sectional dimensions of 5 mm by 10 mm and gauge length of 25 mm were produced and tested five successive times with the measurement taken (modulus) for each sample and the mean value was used as the parameter value. Electrical conductivity and resistivity test was performed on AA1070/SiC specimen with Keithley Instruments Model 2400.   Each specimen with cross-sectional area of 10 mm by 10 mm with length of 100 mm were prepared for testing in 4-point probe set up machine. The working voltage is 20 mV. Voltage, current, resistivity and conductivity were obtained from this setup.
Polarization tests were conducted 37°C with a triple electrode consisting of platinum counter electrodes, Ag/AgCl reference electrodes and embedded AA1070/SiC working electrode with visible surface area of 1 cm 2 with a transparent/aerated cell accommodating 200 mL of the test electrolyte media and connected to Digi-Ivy 2311 potentiostat. Anodic/cathodic potentiodynamic polarization curves were produced at 0.0015V/s scan rate between set potentials of −1.25V and +1.5V. Corrosion current density ( j c ) and corrosion potential (E c ) values were determined using the Tafel extrapolation method. The corrosion rate (C R ) and the inhibition efficiency (η, %) were calculated from the formulae below: j c is the current density (A/cm 2 ), D is the density in (g/cm 3 ); E q is the equivalent weight (g) and 0.00327 is the corrosion rate constant (mm/y). Open circuit potential (OCP) values were acquired at 0.05 V/s step potential for 1,500 s. Optical images of AA1070/SiC morphology was studied after corrosion with Omax trinocular metallurgical microscope using ToupCam analytical software.
The statistical importance of SiC grain size and chloride concentration on the corrosion rate of AA1070/SiC was studied through analysis of variance (ANOVA) at 95% confidence (significance level, α = 0.05) with respect to the following equations.
The sum of squares among columns (NaCl concentration) Sum of squares among rows (SiC grain size) Total sum of squares 3. Result and discussion

Potentiodynamic polarization studies
Anodic-cathodic polarization plots for AA1070/SiC with specific SiC grain size (3µm, 9 µm, 29 µm and 45 µm SiC grain size) in 0.25M H 2 SO 4 solution at 0%, 0.25%, 0.5%, 0.75% and 1% NaCl concentration are shown from Figure 1(a) to 1(e). Results of corrosion rate, corrosion potential, corrosion current density, anodic and cathodic Tafel constants and polarization resistance obtained from the polarization test are shown in Table 4. Observation of AA1070 corrosion at 0 µm SiC grain size in 0.25M H 2 SO 4 /0%, 0.25%, 0.5%, 0.75% and 1% NaCl solution shows a relative increase in corrosion rate of AA1070 with increase in Cl − concentration from 1.185 mm/y at 0% NaCl to 2.251 mm/y at 1% NaCl. AA1070 generally exhibits good corrosion resistance in astringent conditions due to the evolution of a protective oxide on the metal surface. However, Cl − ions tend to cause instability and possible degradation of the passive film, leading to breakdown and pitting corrosion of the metal. The experiment was conducted in aerated containers; thus it can be concluded that the aluminium metal without SiC content was readily polarized in the acid chloride solution. The chlorides probably contributed to the formation of soluble chlorinated aluminium hydroxide in the presence of oxygen, which hindered the formation and or repassivation of the protective oxide.
Addition of 3 µm and 9 µm SiC grain size to AA1070 matrix at 0% NaCl concentration improved the corrosion resistance of AA1070/SiC with relative reduction in corrosion rate in comparison to values obtained at 0 µm SiC grain size. The corrosion rate decreased to 0.386 mm/y and 0.287 mm/ y from 1.185 mm/y as earlier mentioned. Further increase in SiC grain size to 29 µm and 54 µm did not confer a positive effect on the corrosion rate of AA1070/SiC as the corrosion rates increased to 1.479 mm/y and 1.715mm/y, respectively. This observation is probably due to changes in the microstructural and interfacial properties between the enlarged SiC grain size and AA1070 matrix. Furthermore, addition of the increased SiC grain size could lead to coarse discontinuities in the protective oxide film, thereby causing an increase in number of sites prone to corrosion (Aziz, Qi, & Min, 2009;Trowsdale et al., 1996). This assertion is confirmed from the observation of Brinell hardness values on Table 3 which shows that at 3 µm and 9 µm SiC particle size the HV value of AA1070/SiC matrix composite increases from 19.6 HV at 0 µm to 35.2 HV and 34.3 HV, respectively. However, increase in the SiC particle size to 29 µm and 45 µm caused a significant decrease in Brinell hardness value to 26.2 µm, respectively. The higher HV values obtained at 3 µm and 9 µm SiC particle size are due to decrease in porosity within the matrix composite microstructure and hence less discontinuities on the oxide protective film of the composite (Akin et al., 2009;Mashhadi, Khaksaria, & Safi, 2015). Significant cathodic shift was observed on the polarization plots of Figure 1(a) on addition of SiC compound (3 µm, 9 µm, 29 µm and 45 µm SiC grain sizes) to AA1070 in 0.25M H 2 SO 4 /0% NaCl due to dominant hydrogen evolution and oxygen reduction reactions on the composite surface. Cathodic polarization is responsible for the destabilization of the oxide film during the activation step, leading to selective dissolution of AA1070/SiC. This reaction involves alkalinization of the aluminium/solution boundary during oxygen reduction causing chemical dissolution of the oxide layer (Baeck and Frankel, 2003;Serdechnova, Volovitch, Brisset, & Ogle, 2014).  At 0.25%-1% NaCl concentration, the corrosion rates of AA1070/SiC at 3 µm and 9 µm SiC grain sizes are marginally lower than the corrosion rate of AA1070/SiC at 0 µm SiC grain size; however, a proportionate increase in value at these SiC grain sizes (3 µm and 9 µm) was observed with increase in NaCl concentration. As earlier observed, AA1070/SiC at 29 µm and 45 µm SiC grain size significantly reduced the corrosion resistance of AA1070/SiC in 0.25 M H 2 SO 4 /0.25%-1% NaCl. Their corrosion rates were observed to increase with increase in NaCl concentration. Cathodic shift also occurred on the polarization plots of AA1070/SiC [ Figure 1(b)] at 3 µm, 29 µm and 45 µm in 0.25 M H 2 SO 4 /0.25% NaCl solution for reasons earlier discussed; however, from 0.25 M H 2 SO 4 /0.5%-1% NaCl minimal anodic shift was observed on the polarization plots [ Figure 1(c)-(e)] due to increase in dissolution rate of AA1070/SiC resulting from increased Cl − ion concentration. This phenomenon results in extensive localized attack of Cl − ions and is attributed to galvanic action between the enlarged SiC grains (Loto & Adeleke, 2016;Reena, Nayak, & Nityanand, 2016). The corrosion resistance characteristic of AA1070/SiC is associated closely with its SiC grain size, which might be responsible for the limited formation of second-phase precipitates and interaction products. Optical microscopy analysis of AA1070/ SiC morphology (to be discussed later) shows the presence corrosion pits formed on AA1070/ SiC than in AA1070 alloy.

Open circuit potential (OCP) measurement
The thermodynamic stability of AA1070/SiC at 0 µm, 3µm, 9 µm, 29 µm and 45 µm SiC grain sizes in 0.25 M H 2 SO 4 /0%, 0.25% and 1% NaCl are shown in the graphical plots of OCP versus exposure time [ Figure 2(a)-(c)]. OCP values for AA1070/SiC at 0 µm SiC in 0.25 M H 2 SO 4 /0% NaCl [ Figure 2(a)] tend to be more electronegative than AA1070/SiC at higher SiC grain sizes, thus more likely to participate in the electrochemical corrosion reactions in the acid chloride solution without applied potential (Hoar, 1967;Hoar & Jacob, 1967). At 0s, its OCP value (−0.505 mV) decreased to −0.565 mV at 127s due to active electrochemical reactions, before increasing progressively to −0.479 mV at 1,500 s. Similar OCP behaviour was observed for AA1070/SiC at 3 µm SiC, but at higher OCP values. The OCP values for AA1070/SiC at 9 µm, 29 µm and 45 µm SiC grain sizes showed relative thermodynamic stability at much higher potentials.
Addition of 0.25% NaCl [ Figure 2(b)] to the electrolyte solution did not necessarily change the dynamics of the OCP curves compared to Figure 2(a); however, potential transients are visible due to current flow from localized corrosion sites on AA1070/SiC as a result of the presence of Cl − ions (Strehblow, 1984). The potential transients are absent on AA1070/SiC (0 µm SiC) between potentials of −0.555V and −0.529V despite visible change in the OCP curve probably due to electrochemical dynamics associated with repassivation of the oxide layer on the composite (Stellwag, 1998). Potential transients were completely absent from AA1070/SiC (3 µm SiC) despite relative thermodynamic instability throughout the exposure hours due to higher resistance of AA1070/SiC at 3 µm grain size. This observation agrees with the result from the polarization test. At higher SiC grain size, the OCP values shift to noble potentials, indicating higher thermodynamic stability as earlier mentioned. At 1% NaCl [Figure 2(c)], the effect of SiC grain size seems too negligible with respect to thermodynamic stability of AA1070/SiC within the electrolyte with minimal variation between −0.509V (0 µm) and −0.497V (29 µm) at 1,500 s; however, at 45 µm there is a large anodic shift to −0.401V (0s) and −0.459V (1500s). Potential transients are clearly visible through the exposure hours at all SiC grain sizes due to excess Cl − ions. Delineation of the results from OCP shows addition of SiC of different grain size shifts the overall OCP plots of AA1070/SiC anodically. Though it does not necessarily improve the corrosion resistance, it actually improves the thermodynamic stability of the composite in the acid chloride media.

Optical microscopy analysis
Optical microscopic images of AA1070 and AA1070/SiC surface morphologies before and after corrosion at magnification ×40 are shown from Figures 3 to 6(e). Figure 3 shows the general morphology of AA1070 and AA1070/SiC before corrosion test. The morphologies of AA1070 and AA1070/SiC after corrosion in 0.25 M H 2 SO 4 /0% NaCl media [ Figure 4 Second, higher SiC grain size connotes more and deeper micropits on the composite surfaces. Formation of corrosion pits occurs due to adsorption of Cl − ions at discontinuities in the surface oxide film resulting from chemical reaction between the Cl − ions and valence electrons on the aluminium matrix (Foley, 1986). The nonhomogeneous properties of the composite most importantly involving the reaction of SiC and the aluminium matrix dominate the outcome reaction of the composite with its environment (Nunes & Ramanathan, 1995). The visible corrosion features on AA1070/SiC morphology corrosion than AA1070 are due to galvanic action due to lack of continuous surface oxide on the composite (Reena et al., 2016). Microstructural anomalies and processing factors also play a major role because at 29 µm SiC grain size the depth of AA1070/SiC pitted surface seems more shallow than at 9 µm SiC grain size (Gnecco & Beccaria, 1999;Hihara, 2005;Trzaskoma, 1990).  deterioration can be observed with respect to SiC content; however, the extent of deterioration is much more severe for AA1070/SiC at 29 µm and 45 µm SiC grain size [ Figure 6(d), (e)] with deep corrosion pits prevalent in Figure 6(e).  composites improves the corrosion resistance properties of the matrix composite in chloridecontaining environments.

Conclusion
SiC grain size variations significantly influence the corrosion resistance properties and OCP values of AA1070 aluminium matrix composite in acid chloride media. Low SiC grain size had a positive effect on the matrix alloy, while higher SiC grain size increased the corrosion resistance susceptibility of the aluminium matrix due to galvanic action among the intermetallic precipitates. Cathodic corrosion potential shift at low SiC grain size is due to cathodic polarization of the matrix surface, while the anodic shift at higher SiC grain size is due to anodic deterioration of the matrix alloy. SiC grain size tends to be more statistically relevant on the matrix alloy corrosion resistance from ANOVA analysis. Microanalytical studies show pitted morphology of the aluminium matrix for  all SiC grain size at low chloride concentration, while the morphology of the aluminium matrix at higher chloride concentration showed total degradation.